Systems that use information relating to the pressure in the cylinders to control or diagnose the operating state of a diesel engine are known in the art. The accuracy of control or diagnosis depends on the accuracy of the apparatus for acquiring the pressure in the cylinders.
In the conventional way, a cylinder pressure sensor and conditioning means are used to acquire the pressure in a cylinder. The sensor comprises an active component that constitutes the component sensitive to the measured physical magnitude. However, the signal delivered by the sensor generally needs to be processed to obtain a “measurement” signal that is significant of the measured physical magnitude. For example, if the active component of the sensor is a piezo-electric component that is sensitive to a deformation to which it is subjected, the signal that it delivers corresponds to a range of deformation values. The conditioning means are adapted to format this signal and to apply conditioning parameters to make this range of deformation values correspond to a range of pressure values.
Calibration methods that obtain the correspondence between the range of deformation values and the range of pressure values, i.e. that determine the conditioning parameter values, for a family of sensors of the same design are known in the art. This correspondence, which is determined by the manufacturer of the sensor, for example, or by post-production tests on a sample of sensors mounted on motor vehicles, is then stored permanently in the conditioning means throughout the service life of the motor vehicle.
Unfortunately, the values of the characteristics of the sensors are subject to significant spread.
Accordingly, mass-produced sensors are subject to an uncertainty as to their sensitivity that may vary strongly from one sensor to another, and as a consequence there is significant uncertainty for a particular sensor if the values of the conditioning parameters have been determined for a family of sensors.
Also, the characteristics of the sensor are subject to drift during its service life, for example as it wears. Accordingly, at the end of a certain time, the original values of the conditioning parameters are no longer valid and the resulting measurement error is uncertain.